\[p(h, R) = \frac{2\pi R h}{2\pi R^2 + 2\pi Rh}\]
\[p(\eta) = \frac{\eta}{1 + \eta}\]
\[ p(h, R) = \frac{h}{2(2R) + h}\] \[ p(\eta) = \frac{\eta}{4 + \eta} \]
\[ p(\eta) = \frac{\arctan (\eta/2)}{\arctan(\eta/2) + \arctan(2/\eta)} \]
\[ p(\eta) = \,? \]